The present invention relates to a construction of nuclear reactor core.
In general, due to an uneven axial void distribution, the power distribution is skewed to the lower side of the reactor core, in boiling water reactors. At the same time, the power peaking is relatively steep and high.
In order to prevent this power peaking from becoming excessively steep and high, there have been made various attempts up to now. One of these conventional attempts is to shift the positions of all fuel assemblies at the time of fuel reloding (this will be referred to as "shuffling", hereinafter), so as to diminish the radial power peaking of the reactor core. Another attempt is to insert a number of control rods (these rods will be referred to as "shallow rods", hereinafter) from the lower side of the reactor core.
However, the first mentioned countermeasure consisting in shifting the positions of all fuel rods has been found inconvenient in that it considerably prolongs the time required for the fuel reloading. Also, the use of the shallow rods causes such inconveniences that the power is abruptly changed at an area close to the ends of these shallow rods and that a troublesome work becomes necessary for adjusting the depth to which the shallow rods are driven.
Further, the conventional reactor cores have suffered from many drawbacks such as difficulty in planning of the reactor core management, and so on.